The present invention relates to a printing frame for the exposure of photographic material as is known from German Patent Specification No. 2,928,320. Such printing frames serve in the preparation of copies from a master, in particular the exposure of films, offset printing plates or photopolymer printing plates which are to be termed collectively as photographic material, the master and the photographic material together being the copying partners.
In the exposure of such photographic materials through a master, the quality of the copying result depends decisively on a flush, completely space-free contact of the photographic material on the master. To ensure this, in order to press the photographic material reliably against the master over its entire area, in the case of downward exposure the glass plate is swung down onto a pliable mat with the copying partners lying on it, the plate coming into sealed contact with a sealing bead bounding the mat. Through an opening in the mat, which can be connected to a vacuum source, a vacuum is introduced into the space between the glass plate and mat, so that the mat and the copying partners (master and photographic material) resting on it are sucked against the glass plate or pressed by the greater atmospheric pressure acting on the underside of the mat against the glass plate.
The mat is usually a fabric-coated mat which, although it has a low coefficient of friction, is rigid in its plane and not compressible. In the case of such mats, the problem arises of avoiding hollow copies, i.e., copies with unsharp regions which occur due to the presence of spaces which remain between the master and the photographic material, in spite of efforts to achieve a full-area intimate contact. In these space regions, the mat has been kept a distance from the glass plate or the copying partners. One reason for this spacing is that the borders of the mat are usually firmly fixed on the base plate so that the mat is stressed relative to the glass plate. On account of this stress, the borders of the mat cannot creep outwards or to the side as would be necessary if any rising of the mat were to be compensated under vacuum effect. It must also be mentioned that the mats have corresponding tolerances.
In the case of prior art designs, a remedy is sought by having a fold incorporated all around in the vicinity of the borders of the mat parallel to the sealing beads, which fold facilitates the relative shifts occurring between mat and glass plate. The same effect is achieved if a jutting sealing web is provided all around along the borders of the mat, the web deforming in an S-shape whenever the border of the mat creeps. In this way, it is intended to ensure the possiblity of compensating for waves and bumps by permitting the borders to creep still during the laying flat.
The results achieved with the known prior art designs are not, however, always satisfactory because the fold running along the border of the mat quickly draws together when the vacuum is introduced and loses its effect so that the border fixing of the mat again hinders the compensation of risings and thus complete laying flat after all.